A previous in vitro study has shown high accuracy, but no clinically significant differences in a group of five electronic root canal length measuring instruments. The purpose of this study was to evaluate and compare the performance of the same group of instruments under clinical conditions and to correlate their accuracy to radiographic estimates of canal length. Five electronic root canal length measuring instruments were used to measure the working length to the "apex" in 20 single-rooted teeth scheduled for extraction. After extraction, the actual canal length was measured visually to a point just within the apical foramen. This length was compared with instrument length as determined electronically. The accuracy of the instruments in determining canal measurement with ±0.5 mm from the apical foramen varied from 55 to 75%. The differences between the instruments were not statistically significant. On average, all of the instruments except for the Endocater gave canal length measurements that were beyond the apical foramen. The variability of the measurements, which was comparable to that of estimates of canal length from preoperative radiographs, indicated that radiographic verification of the working length is still desirable. © 1990 The American Association of Endodontists.